Development of the Second Day 147 



veins and arteries and the entire wall of the 

 capillaries are derived from the entoblast. 



The first indication of the formation of the 

 blood vessels is seen, on the first day, as a 

 reticulated appearance in that part of the meso- 

 blast surrounding the chick that has been 

 called the vascular area. This network be- 

 comes more distinct during the second day, 

 and begins to show irregular, reddish blotches, 

 which are known as blood islands, because from 

 their cells the blood corpuscles are formed. 

 These changes take place in the splanchnic 

 layer of mesoblast, which is separated, at this 

 time, from the somatic layer by the extension 

 of the body-cavity into the extra-embryonic 

 area of the mesoblast. 



The network is due to the development of 

 what is known as the angioblast, which is a set 

 of cells collected between the mesoblast proper 

 and the endoderm. The cords of the angio- 

 blast are at first solid, but they soon acquire 

 lumena, which, by becoming united, form a 

 continuous though indefinite vessel. The first 

 of these channels to take on a definite form 

 and position is one that forms a sort of circu- 

 lar boundary to the entire vascular area and is 

 known as the simis terminalis (Fig. 56). This 



